As I've previously noted, Batman Beyond exists in a shaky area of continuity. Even so, his comics still continue to be published, and I'll treat them as "counting" until I see beyond a shadow of a doubt that they don't. Luckily, that means we have more stuff to add! (You can see the previous Batman Beyond Super Post here.)
Warren McGinnis first appeared in the first episode of Batman Beyond, "Rebirth" (1999); he's the father of Terry McGinnis, the new Batman. He didn't appear in comics until Batman Beyond #7 (June 2012), even though the rest of Terry's family had. He also has a flashback cameo in Justice League Beyond #17 (January 2013).
Mr. Fixx also first appeared in "Rebirth" (1999) as the assistant/muscle of Derek Powers. He made his comics debut in Batman Beyond #7 (June 2012), where we learn he was behind Warren McGinnis's death. He also appears in Justice League Beyond #17 (January 2013), where we learn how he got his scar. (Spoiler alert: Batman did it.)
Melanie Walker, aka Ten, first appeared in the Batman Beyond episode "Dead Man's Hand" (1999) as a member of the Royal Flush Gang. She later went straight and became friends with Terry McGinnis. She made her comics debut in Batman Beyond 2.0 #10 (October 2013) and became a pretty regular character for the series so far.
Sam Young is a district attorney - and husband of Barbara Gordon - who first appeared in the episode "A Touch of Curare" (1999). He later showed up in Batman Beyond 2.0 #16 (March 2014) in a flashback about Barbara Gordon.
Charlie "Big Time" Bigelow first appeared in the episode "Big Time" (2000), where he was a childhood friend of Terry's that got mixed up with the wrong people. During a heist at Wayne-Powers, he also got mixed up with the wrong chemicals and became what you see below. He later had a cameo appearance in Batman Beyond 2.0 #1 (August 2013).